As these panels from WALT DISNEY'S COMICS AND STORIES # 211 (Dell Comics, Cover Date: April, 1958) clearly show, it doesn't take very much to turn Mickey Mouse into a GHOST!
Just remove his CAP, and have the colorist neglect to color his RED PANTS...
...And, PRESTO! Instant GHOST!
Illustrations are from "The Castaways of Whale Bay" (1958) written and drawn by the classic team of Carl Fallberg and Paul Murry!